I was recently made aware of the fact that I do not have tail lights on my '90 F150 4x4. The officer was nice about it though. Anyway I have the following um..problem, I have brake lights except when the head lights are turned on, rear turn signals will flash both sides but not at all when head lights are on. Front lights and turns work fine. Checked and replaced bulbs, traced wire as much as possible but nothing. I have not gone under the dash yet. I only have 8 days left on the fixit ticket, any ideas.

One more thing, the turn signals flash normal until the lights are turned on then they get very slow and stop at times. Also when turned on and the brake pedal is pressed as far as possible they do the same thing but if you let up a little on the pedal they flash (inside and front) normal.

 

You have a bad ground at the rear tailights. This could be at the socket where the outside part of the bulb contacts the tab on the socket, a problem with the connection inside the socket, or a bad connection where the ground wire bolts to the body. You have to take the whole lense out to see the ground connection.

 

what ever happened on this? having similar problem & would like to know the outcome... thanks, Gary

 

Well it was the connection with the hitch but I am not so sure now. I have had to take the connections apart several times. It works for awhile and then it starts to act up again. Now if I turn the lights on I have tail lights but no break lights but if I tap the break several times quickly I can usually get everything to work. It is still a short but I am not sure where since the front turn signal are never affected.I would say to start at the tail lights and work forward. Sometimes a bad buld will cause the same affect, I replaced all mine. Good luck.

 

I had a very similar problem with strange behavior in the tail and brake lights. it turned out to be a short or something within the bulb socket itself. most all parts stores sell replacement sockets with the crimp connectors already attached. I just replaced both sockets and then everything was working perfectlt again.

 

I have had the Ford sockets fail several times. It always seems to be the ground connection up inside the socket itself. I replaced the socket with a used one and in a couple of years it did the same thing again.

In this instance it was not the ground where the brass part of the bulb meets the tab, and it was not the spot where the black wire bolts to the body, it was the connection in the socket where the black wire is crimped to the metal tab that meets the brass part of the bulb.

If you have the brake lights or turnsignals working with the headlights off, and not working with the headlights on, it's almost always a ground problem with the rear bulbs.

 

In my case it the brake lights work with the lights off, the turn signals work unless the brake lights or head lights are on. With the brake light on both rear turn signals flash slowly at the same time. With the headlights and brake lights on the turn signals will not flash, just stay lit on dash.

Currently I try not to turn much and drive only in the day time. A real hassle.

 

What happens is you have a dual filament bulb for the turn/brake and the running light. You have one wire coming in for the turn/brake function, and another(the brown) for the running light. The ground for both filaments are tied together at the bulb, were they are soldered to the brass part of the bulb.

If this ground is corroded or bad, the current will try to find an easier path back to ground. It will find this easier path by going backwards up the other circuit. In your case, it's finding a path by going backwards up through the running light circuit. So when the running lights are off(the headlights are off) it's working(though not bright as it could) by finding a ground in the running light circuit. But when you turn the headlights on, 12 volts is now coming down the running wiring, and blocks the ground the other circuit was using. So the lights do not work with the headlights on.

If you want to test this theory(strange things happen sometimes) get a scrap piece of wire and bolt one end to a known good ground. Take the taillight assembly out, unplug the the socket, stripe the other end of the scrap wire, and with the turnsignal flasher on, and the headlights on, stuff the bare copper end of the scrap wire down in beside the bulb so it touches the brass part of the bulb. If the bulb starts working, then you know you have a grounding problem

 

well said, Dave. It has been my experience in the past that most of the times lighting problems can be traced to bad grounds no matter where the light is on the truck (or car, or whatever) I never thought of using the extra wire for checking for bad grounds. That tip will surely save me some time and frustration in the future when I am trouble shooting wiring problems. (not IF it happens in the future, simply WHEN) I have way too many older vehicles to not have a problem with one of them soon.

Thats why I love this forum, good people offering good advice and sharing cool tips and tricks. I have learned more on FTE in the past year and a half than I ever learned in a lifetime of books.

Thanks again to all FTE'ers